1. Field of the Invention
The present invention relates to a thermosensitive recording material comprising as the main components a leuco dye serving as a coloring agent and a color developer capable of inducing color formation in the leuco dye upon application of heat thereto.
2. Discussion of Background
There are conventionally proposed various recording materials which utilize the coloring reaction between a colorless or light-colored leuco dye and a color developer capable of inducing color formation in the leuco dye upon application of heat or pressure thereto when brought into contact with the leuco dye.
A thermosensitive recording material, one of the above-mentioned recording materials, is usable as a recording material for an electronic computer, facsimile apparatus, ticket vending apparatus, label printer, and recorder because it has the advantages that complicated processes such as development and image-fixing are not required, recording can be achieved for a short period of time using a relatively simple apparatus, there is no noise development, and the manufacturing cost is low.
In such a thermosensitive recording material, colorless or light-colored leuco dyes having a lactone, lactam, or a spiropyran ring are used as coloring dyes, and organic acids or phenols are conventionally employed as color developers. The thermosensitive recording material using the above-mentioned leuco dye and color developer is widely used for practical use because the produced images have high density, with the whiteness of the background maintained high.
In recent years, in line with diversified usage for the thermosensitive recording materials, there is an increasing demand for the improvement of quality of the thermosensitive recording material. In particular, when great importance is attached to the reliability of the recorded image, the requirements for the preservation stability of the recorded image, specifically, the resistance to oil and plasticizer of the images are increasing. The development of a recording material which can satisfy such requirements is therefore intensively desired.
In addition, a bar code reader employing a semiconductor laser beam is mainly used for the system of point-of-sales (POS). Such a semiconductor laser beam has a wavelength of 650 nm or more, mainly 680 nm or more, from the visible light range to the near infrared range. Therefore, it is necessary that bar code images absorb the visible rays and the near infrared rays within the above-mentioned range.
To improve the preservation stability of the image recorded in the thermosensitive recording material, it is proposed to use as the color developer a phenolsulfone compound, as disclosed in Japanese Laid-Open Patent Applications 58-82788 and 60-13852; a metallic salt of benzoic acid, as disclosed in Japanese Laid-Open Patent Application 61-47292; and a substituted salicylic acid compound, as disclosed in Japanese Laid-Open Patent Application 62-169681. Even though the aforementioned compounds are used as the color developers, the fastness to oils, plasticizers and water of the image areas obtained in the recording materials is not sufficient.
The formation of an intermediate layer is proposed to increase the recording sensitivity. For instance, there are proposed an intermediate layer comprising a variety of inorganic pigments, and an intermediate layer comprising non-expandable void particles. However, these recording materials have the shortcomings that the thermal coloring sensitivity is insufficient and the recording properties are poor.
As previously mentioned, images are formed in the thermosensitive recording material when an electron-acceptor material serving as a coloring developer induces the color formation in a dye upon application of heat thereto. To read such images using the semiconductor laser beam, it is required that the images have absorption intensity in the wavelength of 650 nm or more, preferably 680 nm or more. However, the image formed by the coloring reaction between the conventional leuco dye and phenol compound cannot sufficiently absorb the light with the aforementioned wavelength. In addition to this, the images thus formed in the recording material are lacking in reliability. Therefore, such a conventional thermosensitive recording material is not useful in practice.
As disclosed in Japanese Laid-Open Patent Application 62-85986, modification of the structure of a dye for use in a thermosensitive recording material makes it possible to read the images recorded in the thermosensitive recording material by the semiconductor laser beam. This method, however, is not practical from the viewpoint of manufacturing cost.